Method and apparatus for post-packaging testing of one-time programmable memories

Abstract

A method and apparatus for post-packaging testing of one-time programmable memories provides means for assuring that each cell of the memory will appear to a customer to be erased and that it is capable of being programmed. The preferred embodiment of the invention is a microcomputer including one-time programmable memory, but the invention also includes memory-only devices. Marginal reading method and apparatus provide for detecting the threshold voltage of memory cells below the level at which the cell appears to the customer to be erased and marginal programming method and apparatus provide for slightly increasing the threshold voltage of cells in order to ensure their programmability.

I claim: 
     
       1. A one-time programmable memory comprising a cell having a threshold voltage which is variable within a known range, programming means for increasing said threshold voltage of said cell to a value above a maximum rated supply voltage of said memory and read means for determining whether said threshold voltage of said cell is below an actual supply voltage, wherein said threshold of said cell may not be decreased after said memory is packaged and wherein the improvement comprises: said programming means is operable to increase said threshold voltage in increments which are small compared to said known range; and   said read means is operable to determine whether said threshold voltage is below a value substantially different from said actual supply voltage;   whereby said programming means and said read means may be used to verify that said threshold voltage is capable of being increased without increasing said threshold voltage above a minimum rated supply voltage of said memory.   
     
     
       2. A memory according to claim 1 wherein said programming means further comprises: means for applying a programming voltage to said cell for a first predetermined time, said first predetermined time being chosen to increase said threshold voltage to a value above said maximum rated supply voltage; and   means for applying a programming voltage to said cell for a second predetermined time, said second predetermined time being chosen to increase said threshold voltage in an amount which is small compared to said known range.   
     
     
       3. A memory according to claim 1 wherein said read means further comprises: means for applying a voltage substantially equal to said actual supply voltage to a control gate of said cell;   means for applying a voltage substantially different from said actual supply voltage to said control gate of said cell; and   sense amplifier means for sensing a current flowing through said cell and for determining from said current whether a voltage applied to said control gate is above said threshold voltage, said sense amplifier means having a first operating mode for use when said voltage substantially equal to said actual supply voltage is applied to said control gate and a second operating mode for use when said voltage substantially different from said actual supply voltage is applied to said control gate.   
     
     
       4. A one-time programmable memory device comprising means for receiving a supply voltage in the range of V DDmin  to V DDmax  ; means for receiving a programming voltage; a cell having a variable threshold voltage, said cell being erased when said variable threshold is below V DDmin  and being programmed when said variable threshold is above V DDmax  ; programming means for increasing said variable threshold voltage to a value above V DDmax  ; and reading means for detecting whether said cell is programmed or erased, the improvement comprising: marginal programming means for increasing said variable threshold in increments small compared to the difference between said programmed and erased conditions; and   marginal reading means for determining whether said cell is marginally programmed, said marginal reading means being capable of detecting a marginally programmed condition of said cell at a threshold voltage at which said reading means detects said cell as erased.   
     
     
       5. A memory device according to claim 4 wherein: said reading means and said marginal reading means both comprise a single sense amplifier, said sense amplifier having a high sensitivity range for detecting a first cell current and a low sensitivity range for detecting a second cell current less than said first cell current.   
     
     
       6. A memory device according to claim 5 wherein: said low sensitivity range is used by said marginal reading means and said high sensitivity range is used by said reading means.   
     
     
       7. A memory device according to claim 4 wherein said reading means further comprises means for applying a read voltage to a control gate of said cell and wherein said marginal reading means further comprises: means for applying a special voltage different from said read voltage to said control gate of said cell.   
     
     
       8. A memory device according to claim 4 wherein said programming means further comprises means for applying a programming voltage to said cell for a first predetermined time and wherein said marginal programming means further comprises: means for applying said programming voltage to said cell for a second predetermined time less than said first predetermined time.   
     
     
       9. A memory device according to claim 4 wherein said programming means further comprises means for applying a first programming voltage to said cell for a first predetermined time and wherein said marginal programming means further comprises: means for applying a second programming voltage different from said first programming voltage to said cell for a second predetermined time different from said first predetermined time.   
     
     
       10. A microcomputer comprising a one-time programmable memory device comprising means for receiving a supply voltage in the range of V DDmin  to V DDmax  ; means for receiving a programming voltage; a cell having a variable threshold voltage, said cell being erased when said variable threshold is below V DDmin  and being programmed when said variable threshold is above V DDmax  ; programming means for increasing said variable threshold voltage to a value above V DDmax  ; and reading means for detecting whether said cell is programmed or erased, the improvement comprising: marginal programming means for increasing said variable threshold in increments small compared to the difference between said programmed and erased conditions; and   marginal reading means for determining whether said cell is marginally programmed, said marginal reading means being capable of detecting a marginally programmed condition of said cell at a threshold voltage at which said reading means detects said cell as erased.   
     
     
       11. A microcomputer according to claim 10 wherein: said reading means and said marginal reading means both comprise a single sense amplifier, said sense amplifier having a high sensitivity range for detecting a first cell current and a low sensitivity range for detecting a second cell current less than said first cell current.   
     
     
       12. A microcomputer according to claim 11 wherein: said low sensitivity range is used by said marginal reading means and said high sensitivity range is used by said reading means.   
     
     
       13. A microcomputer according to claim 10 wherein said reading means further comprises means for applying a read voltage to a control gate of said cell and wherein said marginal reading means further comprises: means for applying a special voltage different from said read voltage to said control gate of said cell.   
     
     
       14. A microcomputer according to claim 10 wherein said programming means further comprises means for applying a programming voltage to said cell for a first predetermined time and wherein said marginal programming means further comprises: means for applying said programming voltage to said cell for a second predetermined time less than said first predetermined time.   
     
     
       15. A microcomputer according to claim 10 wherein said programming means further comprises means for applying a first programming voltage to said cell for a first predetermined time and wherein said marginal programming means further comprises: means for applying a second programming voltage different from said first programming voltage to said cell for a second predetermined time different from said first predetermined time.